1. Field of the Invention
This invention relates to semiconductor technology, and more particularly to a method of improving the intrinsic gettering ability of a wafer with a low defect density.
2. Description of the Related Art
Single-crystal silicon wafers are the bases of most of IC fabrications. A single-crystal silicon wafer is usually got by slicing a silicon ingot formed with a Czochralski method. A wafer obtained from the earlier Czochralski method, which is called a CZ wafer in usual, has quite a few micro-defects and is thus not suitable for an advanced semiconductor process with a small critical dimension. Although a method including Si-epitaxy and subsequent annealing has been developed to improve the surface quality of a CZ wafer, the method is quite complicated greatly increasing the production cost.
An advanced Czochralski method was then developed for forming wafers with a very low defect density, wherein the silicon ingot is drawn from a melt of silicon in a lower drawing speed and in a constant ratio of drawing speed to rotation speed. A wafer cut from such a silicon ingot is usually called a Perfect Silicon wafer in the art.
However, since a Perfect Silicon wafer has a very low bulk micro-defect (BMD) density, its intrinsic gettering ability is quite poor so that undesired metal impurities easily diffuse to the active areas significantly affecting the device properties. Thus, it is highly desired to improve the intrinsic gettering ability of a wafer with a low BMD density, such as a Perfect Silicon wafer.